1. Field of the Invention
The present invention relates to high-performance high-nitrogen duplex stainless steels excellent in pitting corrosion resistance.
2. Description of the Related Art
Ni-dependent austenitic stainless steels are used in general corrosive environments and are the grades that occupy the largest market share by taking up about 60% of total stainless steel use. Nickel (Ni) is essentially required as an austenitic phase stabilizer, but due to high price and price fluctuation thereof, it is difficult to keep stable balance between supply and demands. To solve this problem, researchers have actively studied the ways to improve economic feasibility by finding appropriate low-Ni or Ni-free stainless steels as the replacement for the austenite stainless steels.
Duplex stainless steels are the grades that contain finely balanced ferritic and austenitic phase ratio of about 50:50. Compared to the austenite stainless steels, the duplex stainless steels have higher price competitiveness as these need less Ni content, and can be used in a wider range of applications by controlling alloying composition and microstructures. Considering the above-mentioned advantages, studies are conducted on the duplex stainless steels as the replacement for the Ni-dependent stainless steels.
Among the duplex stainless steels, lean duplex stainless steels with further reduced Ni content have been researched and developed, and some grades have been commercialized as the replacements for the previously-used austenitic stainless steels. Examples of lean duplex stainless steels that have been developed are: lean duplex 2304 (UNS 532304) containing 23% chromium (Cr), and 4% nickel (Ni), and LDX2101 (21% Cr, 1% Ni, UNS 532101) having reduced Ni content (down to 1%), but comparable corrosion resistance to that of AISI 316 L stainless steels and even better strength and elongation level than AISI 316 L.
However, because the duplex stainless steels contain approximately 50 vol % ferritic phase which has nitrogen solubility as low as 0.04% by weight or lower, it is not easy to increase nitrogen (N) content in the stainless steels. As the nitrogen solid solution in the basic materials of the duplex stainless steel is firstly incorporated in the austenitic phase, chemical composition of austenitic and ferritic phases deviates from equilibrium due to presence of excess of nitrogen (N) solid solution in the austenitic phase. Further, the formation of Cr—N bonding and precipitates are detrimental to mechanical-chemical properties of the stainless steels. The above drawbacks confine further development and commercialization of high-N duplex stainless steels that actively utilize nitrogen (N), and an appropriate solution is necessary.
Accordingly, after studying the duplex stainless steels having ferritic and austenitic phase having variable range of properties, the present inventors developed compositions for low Ni, high N duplex stainless steels, which utilize manganese (Mn) and nitrogen (N) to stabilize the austenitic phase and thus reduce or eliminate Ni use, and utilize molybdenum (Mo) and tungsten (W) to provide corrosion resistance that is comparable to, or greater than that of the currently used austenitic stainless steels and duplex stainless steels, and have optimum combination of alloying elements to thus exhibit greatly improved mechanical characteristics than the currently available austenitic stainless steels and duplex stainless steels.